Re-Envisioning the Telescope — Centauri Dreams

Image: Diffraction at work. The grooves of a compact disc can act as a grating and produce iridescent reflections.
Credit: Wikimedia Commons.

Re-Envisioning the Telescope

by PAUL GILSTER on NOVEMBER 6, 2012

An inventor named Tom Ditto has been casting a serious look at diffraction gratings as large primary collectors for telescopes, work that has been getting a bit of a buzz on the Internet. See, for example, An Old Idea Gives Telescopes a New Twist, and ponder how much the eponymous Dittoscope sounds like something out of a Tom Swift story. Nonetheless, an instrument based on a diffraction grating as its primary light-gathering source may prove useful in a variety of astronomical settings, including the ability to produce extremely high quality spectroscopic information for radial velocity exoplanet searches.

Diffraction happens when a small obstacle or opening causes a wave of light to interfere with itself, creating patterns that depend on the size of the diffracting object and the size of the wave. A diffraction grating, in this case a flat surface with a regular pattern of grooves, can be used to separate different wavelengths of light, which will interfere at different angles. The use of diffraction gratings in astronomy dates from 1786, when the American clockmaker and astronomer David Rittenhouse ran experiments on the behavior of diffracted light. Joseph von Fraunhofer would later use dispersion by diffraction grating to resolve atomic lines of sunlight and starlight. Spectroscopy has made use of diffraction gratings to study the stars ever since.

In a presentation to the NASA Institute for Advanced Concepts back in 2007, Ditto showed that his Dittoscope could be conceptualized as a standard telescope capable of spectroscopy aimed at a flat grating. The standard telescope’s spectrometer has its own grating and slit. The use of the second spectrometer eliminates the overlapping spectra from the flat primary grating. Each object is thus imaged at a single wavelength at any unique angle of incidence. There are no moving parts other than the rotating Earth, with the instrument oriented east to west.

The ground based Dittoscope, then, takes advantage of the Earth’s rotation, as described in the presentation for Ditto’s Phase I study: “The precession of objects in the night sky causes their incident angles to rotate. For any incident angle there is a corresponding wavelength, so an entire spectrogram can be assembled over the course of a night.” With this enormous field of view — a 40 degree arc — millions of stars are placed within view simultaneously: